Intel 815 manual Source Constant Alpha Blend Enable

Intel® 815 Chipset: Graphics Controller PRM, Rev 1.0

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Bit	Description
31	Source Constant Alpha Blend Enable.
	1 = Enable Source Alpha Blending when the logical OR of the Source Key Mask Enables are asserted
	within the Alpha Blend Window, and the comparison indicates that the overlay is to be displayed.
	0 = Disable Source Alpha Blending.
30:27	Reserved.
26:24	Source Key Mask Enables. Each bit enables one channel. If the bit is a 1, the comparison result is
	used; otherwise, it is ignored.
	Bit 2 = Enables Comparison [23:16]
	Bit 1 = Enables Comparison [15:8]
	Bit 0 = Enables Comparison [7:0]
23:16	Constant Alpha Red [7:0]. This involves three 4 x 8 bit alpha multipliers to be inserted in the Overlay
	and Primary Display merging logic. When the alpha blend is selected, the most significant 4 bits of
	each alpha channel is used as the alpha term. If the original 8 bits of the alpha channel = FF, the alpha
	channel value is treated as a 1. If alpha = 0, 1-0 must also be treated as a 1. This functionality only
	works for a primary display of 16 and 24 bits per pixel.
	pixel (R) = (alphaR * primary display (R)) + ((1-alphaR) * overlay (R))
15:8	Constant Alpha Green [7:0]. This involves three 4 x 8 bit alpha multipliers to be inserted in the
	Overlay and Primary Display merging logic. When the alpha blend is selected, the most significant 4
	bits of each alpha channel is used as the alpha term. If the original 8 bits of the alpha channel = FF,
	the alpha channel value is treated as a 1. If alpha = 0, 1-0 must also be treated as a 1. This
	functionality only works for a primary display of 16 and 24 bits per pixel.
	pixel (G) = (alphaG * primary display (G)) + ((1-alphaG) * overlay (G))
7:0	Constant Alpha Blue [7:0]. This involves three 4 x 8 bit alpha multipliers to be inserted in the Overlay
	and Primary Display merging logic. When the alpha blend is selected, the most significant 4 bits of
	each alpha channel is used as the alpha term. If the original 8 bits of the alpha channel = FF, the alpha
	channel value is treated as a 1. If alpha = 0, 1-0 must also be treated as a 1. This functionality only
	works for a primary display of 16 and 24 bits per pixel.
	pixel (B) = (alphaB * primary display (B)) + ((1-alphaB) * overlay (B))

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